protobuf generation broken

[brettmc]

Recently, our protobuf generation process (make protobuf) has stopped working, due to the image we use (socialpoint/protobuf-tools) no longer being available on docker hub.
It looks like https://docs.buf.build/introduction is the new way to generate APIs, so this could be investigated as a possible solution.

Steps to reproduce
Run make protobuf

What is the expected behavior?
Regenerated protobuf API files are produced

What is the actual behavior?
Failed to pull image

[brettmc]

It looks like the opentelemetry-proto repo is already set up to generate language-specific protobuf code... make gen-php looks to generate sensible output, and it might be as simple as swapping our current build process over to that.